RTC: [Interface] Collaborative editors can elevate capabilities

[chriszarate]

In a collaborative editing session, editors are treated as peers and can effectively elevate their capabilities to that of the person who clicks save.

The primary example is the unfiltered_html capability. If an Admin and a Contributor are collaborating on a post, the Contributor could add HTML to the post that would normally be stripped on save. But if the Admin hits save and continues to publish, the HTML is preserved via their elevated capabilities.